Chlorine is produced almost entirely by electrolytic methods, primarily from aqueous solutions of alkali metal chlorides. In the electrolysis of such solutions or brines chlorine is generated at the anode and an alkali metal hydroxide, such as sodium or potassium hydroxide, is produced at the cathode, together with hydrogen. Because the anode and cathode products should be kept apart to prevent reactions between them many cell designs have been developed to accomplish such separation. These designs have generally utilized either a diaphragm or a mercury intermediate electrode to separate the anolyte and catholyte.
In diaphragm cells brine is fed continuously into the cell and flows from the anode compartment through a diaphragm, such as an asbestos diaphragm, into the catholyte compartment, which contains, for example, an iron cathode. To minimize back-diffusion and migration, the flow rate is maintained so that only part of the salt present is electrolyzed. The hydrogen ions form hydrogen gas at the cathode, leaving hydroxyl ions there. The catholyte solution, which contains sodium hydroxide and unchanged sodium chloride, is subsequently evaporated to obtain the hydroxide. In the course of such evaporation much of the sodium chloride precipitates and is separated, redissolved and sent back to the electrolytic cell, often as an aqueous solution or brine feed to the anolyte compartment. The function of the diaphragm is to maintain a desirably high level of concentration of alkali in the catholyte, to minimize the diffusional migration of hydroxyl ions into the anolyte and to maintain separation of chlorine from hydrogen and alkali metal hydroxide. The diaphragm should also have minimal electrical resistance to lower the cost of power consumed in the electrolysis.
In the mercury electrode process, the cation, usually sodium ion, after conversion to metal at the cathode forms an alloy or amalgam with mercury. The amalgam flows or is pumped to a separate chamber in which it is allowed to undergo reaction, most often with water, to form hydrogen and a comparatively strong sodium hydroxide solution containing almost no sodium chloride.
The diaphragm process is inherently cheaper than the mercury process but because it does not provide chloride-free alkali additional processing steps are necessary when purified and/or concentrated alkali metal hydroxide is the desired product.
In an effort to improve the separating ability of the separating component of diaphragm cells it has been suggested that ion-exchanging membranes should be used in place of the diaphragms. Numerous membrane materials have been suggested and tried. For example, such membranes are described in U.S. Pat. No's. 2,636,851, 2,967,807 and 3,017,338 and in British Pat. No's. 1,184,321 and 1,199,952.
Such membranes are substantially impervious to hydraulic flow. During operation brine is introduced into the anolyte compartment, wherein chlorine is liberated. Then, in the case of a cation permselective membrane, sodium ions are transported across the membrane into the catholyte compartment. The concentration of the relatively pure caustic produced in the catholyte compartment is determined by the amount of water added to this compartment from an external source and by migration of water in the cell, i.e., by osmosis and/or electro-osmosis. While operation of a membrane cell has many theoretical advantages, its commercial application to the production of chlorine and caustic has been hindered, sometimes because of erratic operating characteristics thereof. A number of disadvantages has been noted when these membranes are used, including a relatively high electrical resistance, oxidative degeneration and poor permselectivity, as well as their relatively high cost.
As an alternative to asbestos diaphragms and impermeable ion exchange membranes the industry has sought a suitable porous or microporous plastic diaphragm material. Such a material is a thin electrically conductive chemically resistant plastic sheet having the desired porosity. Numerous references may be found relating to such membrane materials. Mention may be made more particularly of the following patents which employ techniques of compression preforming followed by fritting or sintering, coagulation and deposition on a support.
French Pat. No. 1,491,033, of Aug. 31, 1966, relates to a process for manufacturing porous diaphragms by mixing a solid additive in particulate form into an aqueous dispersion of polytetrafluoroethylene in the presence of particulate inorganic fillers, coagulating the dispersion, forming the resultant coagulum into sheet form and removing the particulate solid additive from the sheet. The removable particulate additive generally is starch or calcium carbonate and is removable by immersion of the resultant sheet in hydrochloric acid. Alternatively, the additive may also be a plastic polymer or other suitable material which is soluble in an organic solvent, depolymerizable, evaporable or otherwise removable upon heating of the sheet or leaching thereof. Particulate inorganic fillers which are suitable include barium sulfate, titanium dioxide and asbestos.
U.S. Pat. No. 3,890,417, issued June 17, 1975, teaches a method of manufacturing a porous diaphragm comprising preparing an aqueous slurry or dispersion comprising polytetrafluoroethylene and a solid particulate additive, thickening the aqueous slurry or dispersion to effect agglomeration of the solid particles therein, forming a dough-like material containing sufficient water to serve as a lubricant in subsequent sheet forming operations, forming a sheet of desired thickness and removing the solid particulate additive from the sheet. The solid particulate additive can be any which is substantially insoluble in water, but which is removable by a suitable chemical or physical means. Examples given are starch and calcium carbonate.
U.S. Pat. No. 3,281,511, issued Oct. 25, 1966, discloses preparing microporous polytetrafluoroethylene resin sheets by mixing a fine polytetrafluoroethylene powder with a carrier and a readily removable filler material and rolling the resulting dough with intermediate reorientation so that the particles are biaxially oriented. The solvent is then evaporated and the polytetrafluoroethylene is sintered at above its melting temperature, followed by removal of the filler with an appropriate solvent. The carrier material is a readily vaporizable material, such as a naphtha or petroleum distillate, e.g., Stoddard solvent, which is a standard petroleum distillate having a flash point not lower than 38.degree. C., consisting mostly of saturated hydrocarbons.
U.S. Pat. No. 3,556,161, issued Jan. 19, 1971, relates to polytetrafluoroethylene sheet materials formed by the "slipforming" process, comprising mixing polytetrafluoroethylene powder with a liquid, such as kerosene, and then sequentially working the resultant composition by the application of concurrent compressive stress and shear stress, the sequence of operations being directed so that the shear stress components are distributed substantially biaxially, resulting in planar orientation in the resulting article. As is the case with the material of U.S. Pat. No. 3,281,511, the sheet material formed is biaxially oriented.
Although these and other well known techniques may result in the production of useful "diaphragm" or microporous sheet materials, in the case of products that are desirably rich in polytetrafluoroethylene they have not been capable of producing membranes of satisfactory mechanical properties, i.e., proper porosity and good wettability.
It is an object of the present invention to provide a novel and improved method of making microporous separators suitable for use in electrolytic cells. It is also an object of the invention to provide an improved separator for use in chlor-alkali cells and fuel cells, which has a low electrical resistance and behaves like a porous medium having a desirable porosity which permits both the passage of electric current and the uniform and controllable flow of electrolyte from one compartment of a cell to another.